The antiapoptotic protein Api5 and its partner, high molecular weight FGF2, are up-regulated in B cell chronic lymphoid leukemia

The apoptosis inhibitor-5 [Api5; antiapoptosis clone 11 (AAC-11); fibroblast growth factor-2 (FGF2)-interacting factor (FIF)] is a 504-aa nuclear protein frequently up-regulated in tumor cells. Several studies have shown potent antiapoptotic action of Api5, which appears to be, at least in part, mediated via suppression of apoptosis depending on the E2F family of transcription factors [1,2,3,4]. Human FGF2 is a prototypic member of the FGF family of growth factors and exists in several variants generated by alternative translation of a single transcript. The smallest variant, 18 kDa FGF2, is released from cells and acts through binding and activation of cell-surface FGF-receptor tyrosine kinases. In contrast, the high molecular weight (HMW) FGF2 variants (22, 22.5, and 24 kDa) localize predominantly to the nucleus and have an intracrine, FGF receptor-independent mode of action [5]. It is interesting that HMW but not 18 kDa FGF2 interacts with Api5 in mammalian cells [3]. Similar to Api5, HMW FGF2 has a prosurvival action in various experimental models [6], although it is unknown to what extent HMW FGF2 and Api5 depend functionally on each other.